Hi Rashmikants,
Please save Your City Data in ViewState then on Button Add Click event when u add new DropDownlist Bind this data to That Drop DownList.
eg.
ViewState["City"]=dataTable;
protected void ButtonAdd_Click(object _object, EventArgs eventArgs )
{
DropDownList dropDownList =new DropDownList();
dropDownList.DataSource=(DataTable)ViewState["City"];
dropDownList.DataBind();
}
:) Enjoy